1. Field of the Invention
The present invention relates to a laser beam control device, and more particularly to a laser beam control device for use in laser beam printers which is capable of optically adjusting the intensity of a laser beam, i.e. the density of images, on the image forming plane or surface.
2. Description of the Prior Art
Laser beam printers are generally adapted to project a laser beam on a photosensitive material providing an image forming surface to form images thereon. Such laser beam printers must have the function of shifting the intensity range of the laser beam, i.e. the function of varying the image density, on the image forming surface to offset the variations in the performance of laser light sources from lot to lot or in the sensitivity of photosensitive material from lot to lot, and to meet the needs of the user. The intensity range of the laser beam is shiftable on the image forming surface by adjusting the input signal to be fed to the drive circuit for driving the laser beam light source (such as a laser diode) and thereby varying the intensity of the laser beam to be emitted by the light source, but this method has the drawback of necessitating a complex video signal generating circuit for feeding the signal to the drive circuit. Accordingly, some conventional printers are adapted to shift the intensity range of the laser beam on the image forming surface by providing a polarizing filter or the like in the path of the laser beam from the light source to the image forming surface and rotating the filter or the like about its optical axis, without varying the intensity of the laser beam to be emitted by the light source.
FIG. 6 shows the principle of shifting the light intensity range by a polarizing filter. Indicated at 61 in the drawing is a laser diode used as the laser light source, and at 62 a polarizing beam splitter serving as the polarizing filter. The laser beam emitted by the laser diode 61 is linearly polarized in a direction parallel to the active layer 61a of the laser diode 61. The polarizing beam splitter 62 has the characteristics of transmitting a component P of the incident beam (the component indicated at 4 in the drawing and having an electric field vibrating in a plane perpendicular to a reflecting plane 62a) and reflecting a component S (indicated at 5 and having an electric field vibrating in a plane parallel to the reflecting plane 62a). Accordingly, when the beam splitter 62 as positioned to give the maximum transmittance is rotated about its optical axis, the intensity of the laser beam passing through the splitter 62 periodically varies with the angle of rotation of the splitter 62. The amplitude of the laser beam through the beam splitter 62 is in proportion to cos .theta. wherein .theta. is the angle of rotation of the splitter 62 from a reference position. The intensity of the transmitted beam is proportional to cos.sup.2 .theta.. FIG. 7 is a graph showing the relation of the rotational angle of the beam splitter 62 to the intensity of the transmitted beam, as well as to that of the reflected beam. In this graph, the intensity of the transmitted beam is indicated in a solid line, and that of the reflected beam in a broken line.
FIG. 8 is a block diagram of a control circuit for driving a laser beam splitter included in the conventional laser beam printer employing the above method. The polarizing beam splitter 62 is embedded in the center of a rotatable base 81 in the form of a disk. A laser beam LB propagating toward the plane of the drawing is incident on the laser beam splitter 62, and the transmitted light is projected on a photosensitive material to form an image thereon. The base 81 is formed with a hole 82 showing a reference position (generally the position of .theta.=0 in FIG. 6) of the splitter 62. When the beam splitter 62 is in the reference position, the hole 82 is detected by a photosensor 83, which feeds a detection signal to a CPU 84 for the CPU 84 to detect that the beam splitter 62 is in the reference position. The CPU 84 feeds an enable signal to a motor drive circuit 85, thereby allowing the operation of the circuit 85. A stepping motor 86 is connected to the output side of the drive circuit 85, and a density dial 87 on the operation panel to the input side thereof. A drive gear 88 mounted on the output shaft of the stepping motor 85 is in mesh with teeth 89 formed along the periphery of the disklike base 81 for rotating the beam splitter 62 about its optical axis to vary the intensity of beam on the image forming surface. On the other hand, the density dial 87 on the operation panel can be manipulated to adjust the image density from a minimum to a maximum in 8 to 16 steps. In accordance with a particular density selected, the dial 87 feeds a signal to the motor drive circuit 85. In response to the signal from the density dial 87, the circuit 85 drives the motor 86 to adjust the intensity of the transmitted laser beam through the splitter 62 to give the desired image density.
FIG. 9 is a flow chart showing the operation process of the conventional laser beam printer of FIG. 8. When the power supply for the printer is turned on, a self-diagnosis is executed (step S1) for the printer itself to check the operation and the state of its components. As a procedure for the self-diagnosis, the motor drive circuit 85 rotates the beam splitter 62 to bring the splitter 62 to the reference position. The self-diagnosis, when completed, renders the printer ready for printing. The density dial 87, etc. on the operation panel are manipulated to give input signals (step S2). Subsequently, preparation of a photosensitive material, preparation of video input from the host computer, etc. are completed. When depression of the print button by the user is thereafter detected (step S3), the CPU 84 feeds an enable signal to the motor drive circuit 85, which in turn rotates the polarizing beam splitter 62 in accordance with the setting of the density dial 87 (step S4). The laser diode is then driven in accordance with the video signal to form an image on the photosensitive material (step S5). Step S6 follows for the development of the image bearing photosensitive material and other procedures.
With the conventional laser beam printer described above, the rotation of the beam splitter is so controlled that the beam splitter is merely rotated through a specified angle according to the value set by the operator with the density dial. Because of this mode of control which is so-called open-loop control, the operator must adjust the density dial in view of the performance of the laser light source which differs from lot to lot or the sensitivity of the photosensitive material which also differs from lot to lot, hence a cumbersome procedure.
Further semiconductor lasers (e.g., laser diodes) emit a laser beam the intensity of which varies with variations in the temperature of the laser itself, so that the laser beam printer incorporating a semiconductor laser as the laser light source requires the adjustment of the dsenity dial every time the ambient temperature or the like varies. This makes the above-drawback more serious.
Further in the case of color laser beam printers for forming color images using a plurality of laser light sources, the light sources must be provided with respective beam splitters. The conventional method of controlling the rotation of the beam splitter in the open-loop mode described above then has the drawback that the color balance can not be corrected delicately.